Yale Bulletin and Calendar

October 5, 2007|Volume 36, Number 5

Yale team achieves major advances
in quantum computers

Two major steps toward putting quantum computers into real practice — sending a photon signal on demand from a quantum bit (or qubit) onto wires and transmitting the signal to a second, distant qubit — have been achieved by a team of scientists at Yale.

The accomplishments were reported in sequential issues of Nature on Sept. 20 and Sept. 27. In fact, the Yale research is highlighted on the cover of the latter issue, along with complementary work from a group at the National Institute of Standards and Technologies.

Over the past several years, the research team of Professor Robert Schoelkopf in applied physics and Professor Steven Girvin in physics has explored the use of solid-state devices resembling microchips as the basic building blocks in the design of a quantum computer.

First step to making quantum computing useful

Previously, information on a quantum computer had only been transferred directly from qubit to qubit in a superconducting system. Schoelkopf and Girvin’s team has engineered a superconducting communication “bus” to store and transfer information between distant quantum bits, or qubits, on a chip. This work, according to Schoelkopf, is the first step to making the fundamentals of quantum computing useful.

The first breakthrough reported is the ability to produce on demand — and to control — single, discrete microwave photons that can carry encoded quantum information. The sources of microwave energy — which is used in cell phones and ovens — usually do not produce just one photon. This new system creates a certainty of producing individual photons, note the researchers.

“It is not very difficult to generate signals with one photon on average, but, it is quite difficult to generate exactly one photon each time,” explain postdoctoral associates Andrew Houck and David Schuster, who are lead co-authors on the first paper. “To encode quantum information on photons, you want there to be exactly one.”

According to Schoelkopf, “We are reporting the first such source for producing discrete microwave photons, and the first source to generate and guide photons entirely within an electrical circuit.”

In order to successfully perform these experiments, the researchers had to control electrical signals corresponding to one single photon. (In comparison, a cell phone emits about 1023 — or 100,000,000,000,000,000,000,000 — photons per second.) Further, because of the extremely low energy of microwave photons, the Yale scientists had to use highly sensitive detectors and experiment temperatures just above absolute zero.

“In this work we demonstrate only the first half of quantum communication on a chip — quantum information efficiently transferred from a stationary quantum bit to a photon or ‘flying qubit,’” says Schoelkopf. “However, for on-chip quantum communication to become a reality, we need to be able to transfer information from the photon back to a qubit.”

This is exactly what the researchers reported in the second breakthrough. Postdoctoral associate Johannes Majer and graduate student Jerry Chow, lead co-authors of the second paper, added a second qubit and used the photon to transfer a quantum state from one qubit to another. This was possible because the microwave photon could be guided on wires — similarly to the way fiber optics can guide visible light — and be carried directly to the target qubit.

“A novel feature of this experiment is that the photon used is only virtual,” say Majer and Chow, “winking into existence for only the briefest instant before disappearing.”

To allow the crucial communication between the many elements of a conventional computer, engineers wire them all together to form a data “bus,” which is a key element of any computing scheme. Together the new Yale research constitutes the first demonstration of a “quantum bus” for a solid-state electronic system. This approach can in principle be extended to multiple qubits, and to connecting the parts of a future, more complex quantum computer.

However, Schoelkopf likened the current stage of development of quantum computing to conventional computing in the 1950s, when individual transistors were first being built. Standard computer microprocessors are now made up of a billion transistors, but first it took decades for physicists and engineers to develop integrated circuits with transistors that could be mass produced.

Schoelkopf and Girvin are members of the newly formed Yale Institute for Nanoscience and Quantum Engineering, a broad interdisciplinary activity among faculty and students from across the university. Further information and FAQs about qubits and quantum computing are available online at www.eng.yale.edu/rslab.

Other Yale authors involved in the research are J.M. Gambetta, J.A. Schreier, J. Koch, B.R. Johnson, L. Frunzio, A. Wallraff, A. Blais and Michel Devoret. Funding for the research was from the National Security Agency under the Army Research Office, the National Science Foundation and Yale University.

— By Janet Rettig Emanuel

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